A thinly built, 5-foot-7 white man entered the post office around 9 a.m., threatened a clerk with a knife, then fled with an undisclosed amount of cash, according to authorities, who continue to investigate.

"Nobody's been caught yet," a Palmer police spokeswoman said around 4 p.m. Saturday. The clerk was not injured in the incident, the spokeswoman said.

Armed robbery of a U.S. Postal Service facility is a federal crime punishable by up to 25 years in prison.

The suspect was wearing a gray ski mask and a gray-and-white hooded sweatshirt at the time of the incident. The post office is located across from Thorndike Playground near the corner of Church and Main streets.

Also involved in the probe are troopers from the Massachusetts State Police Crime Scene Services Section and the U.S. Postal Inspection Service, the latter of which did not immediately return a phone message from a reporter.

The Postal Service is offering a reward for any information leading to the arrest of the suspect. Anyone with information is asked to call Palmer police at (413) 283-8792 or the Postal Inspection Service at 1-877-876-2455.